Weston cell

The Weston cell, invented by Edward Weston in 1893, is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters. It was adopted as the International Standard for EMF in 1911.

Chemistry

The anode is an amalgam of cadmium with mercury, the cathode is of pure mercury, the electrolyte is a solution of cadmium sulfate octahydrate and the depolarizer is a paste of mercurous sulfate.

As shown in the illustration, the cell is set up in an H-shaped glass vessel with the cadmium amalgam in one leg and the pure mercury in the other. Electrical connections to the cadmium amalgam and the mercury are made by platinum wires fused through the lower ends of the legs.

Characteristics

The original design was a saturated cadmium cell producing a convenient 1.018638 Volt reference and had the advantage of having a lower temperature coefficient than the previously used Clark cell. (Reference cells must be applied in such a way that no current is drawn from them.)

The temperature coefficient can be reduced by shifting to an unsaturated design, the predominant type today. However, an unsaturated cell's output decreases by some 80 microvolts per year, which is compensated by periodical calibration against a saturated cell.

Early Research

Dr. F.A. Wolff, NBS Researcher and 1st Chair of the Washington AIEE Section contributed significant research to the studies of the Weston Cell. Some of these early effort are archived on the IEEE Global History Network Site -> [1]

Dr. Wolff's own report presented in the *Bulletin of the Bureau Standards*, November 1908, volume 5, number 2, pps.309-337, located here: [2]

Also available is an article from the US Department of Commerce and NBS regarding the usefulness of Weston cells: "Standard Cells, Their Construction, Maintenance, and Characteristics" by Walter J. Hamer, National Bureau of Standards Monograph 84, January 15, 1965. This article is also online at the NIST Calibration Services website:[3]

Patent

• U.S. Patent 494,827, "Voltaic cell"

Sources

• Practical Electricity by W. E. Ayrton and T. Mather, published by Cassell and Company, London, 1911, pp 198-203

External links

• Standard Cells
• Special-purpose batteries

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